1. Field of the Invention
The present invention is related to the field of geophysical exploration and more specifically to apparatus and methods of using a seismic receiver in a drill string in a wellbore to acquire seismic data while tripping the drill string from the wellbore.
2. Description of the Related Art
In drilling a borehole to recover oil from the earth, it is often helpful to turn or steer the downhole drill bit toward or away from subterranean targets. To facilitate this geophysical steering, drillers need to know drill bit location on the seismic section. The location of targets ahead of the bit is also required, as well as some warning or indication of drilling hazards such as over-pressured formations or thin, shallow gas intervals. Surface seismic surveys generally include this information, but resolution and depth location is poor because surface seismic surveys are time based (rather than depth based). For example, to determine the depth of a reflection, a speed of sound for the formation must be known. Consequently, these systems require depth calibration to accurately determine locations of target horizons or drilling hazards. Traditionally, this calibration has been provided by either offset well sonic data or wireline checkshot data in the current well. Offset data is often inadequate however due to horizontal variations in stratigraphy between wells.
During surface seismic surveys, a plurality of seismic sources and seismic receivers are placed on the surface of the earth. The seismic sources are triggered in a predetermined sequence, resulting in the generation of seismic waves. These seismic waves travel downward through the earth until reflected off some underground object or change in rock formation. The reflected seismic waves then travel upward and are detected at the seismic receivers on the surface. One or more clocks at the surface measure the time from generation of the seismic waves at each source to the reception of the seismic waves at each receiver. This gives an indication of the depth of the detected object underground. However, the exact speed of sound for these seismic waves is unknown, and thus, the exact depth of the detected object is also unknown. To more closely measure the exact speed of sound, a “wireline checkshot” may be used to calibrate depth measurements. During a “wireline checkshot,” a receiver on a “wireline” is lowered a known distance into an already-drilled borehole. A surface seismic source is then triggered and the time is measured for the seismic wave to travel to the wireline receiver. Because the depth of the wireline receiver is known, an average interval velocity indicating the average speed of the seismic wave can be determined with some degree of accuracy. Wireline checkshots, however, require removing the bit out of the hole, commonly known as tripping, and are often prohibitively expensive.
Systems and tools are known in the art for detecting and storing seismic signals downhole for retrieval and processing on the surface. U.S. Pat. No. 5,555,220 to Minto, assigned to the assignee of this application and incorporated herein by reference, describes a seismic receiver deployed to the bottom of a drill string on a slick line for taking seismic survey data. Seismic data is received and stored and the receiver is retrieved to the surface. A clock in a surface controller is synchronized with a clock in the deployed receiver. The source data is time-stamped using the surface clock. The received data is time-stamped using the downhole clock. The accuracy of the resulting seismic profile is dependent upon the accurate synchronization of the clocks. The downhole clock, in particular, is susceptible to drift caused by substantial changes in temperature found in the downhole environment.
Typical deployment times of such tools is 12-48 hours. This fact translates into a need for clock stability better than 1×10−8 over the deployment time. Common downhole clocks that use piezoelectric crystal oscillators tend to drift with temperature and age, and rarely exceed a stability of 1×10−7. Such clocks commonly use a single oven to control the oscillator temperature. As such, the downhole clock drifts out of synchronization with the surface clock, causing unacceptable degradation of the output seismic profile data.
There is a demonstrated need for an improved clock that is resistant to operationally-induced error and drift for use in downhole systems including downhole seismic systems.